The invention relates to submergible pump installations used, for example, in wells when artificial lift is required, for pumping well liquid to the surface.
Downwell submergible pump installations generally comprise a pump, commonly of the centrifugal type, and ancillary equipment such as a pump discharge head, an electric drive motor for the pump, and a motor protector, for example. The entire installation may be suspended in a well by tube or cable, and the pump may operate directly in a well casing, or in a liner contained within the casing. In either case, when operated, the pump draws in well liquid from beneath the installation and discharges it into the casing or liner through the discharge head. A suitable packoff or seal separates the low-pressure inlet side of the casing or liner from the high-pressure discharge side. When suspended by a tube, the pump may discharge into this tube.
When the pump has operated continuously for a period sufficient for liquid to be pumped from the well, the well tubing (casing or liner) above the pump installation is filled with liquid, so that the pump operates against a back pressure created by this height of liquid. On start-up, however, there may not be a sufficient height of liquid in the well tubing above the pump to create a back pressure of sufficient magnitude for the pump to operate satisfactorily. Insufficient back pressure may, for example, produce excessive flow rates creating vibrations which can cause rotating seals to leak.
To provide sufficient back pressure for a downwell pump on start-up, previously, the well tubing has, for example, been filled by external means, with a control valve provided at the surface. This procedure however has not always been practical or possible, resulting in extended periods of pumping at high flow rates before control can be effected by a surface valve.
The present invention provides means for use in controlling a submergible pump installation so as to maintain operation of the pump against a suitable back pressure.